Hotspot Magmatism Research Articles

Mackenzie igneous events, Canada: Middle Proterozoic hotspot magmatism associated with ocean opening

U Pb ages have been obtained for the Muskox intrusion and the Mackenzie dyke swarm. The age of a pyroxenite from the layered series of the intrusion is 1270 ± 4Ma. Baddeleyite fractions from four widely-spaced Mackenzie diabases define a single discordia line with an upper intercept age of 1267 ± 2Ma. The dyke age of 1267 Ma provides a precise time-marker for much of the northwestern Canadian Shield. Mackenzie intrusive events were coeval with eruption of Coppermine River flood basalts in the Coppermine homocline. The short time-span, large volume and specific focus of Mackenzie igneous events suggest that magmatism occurred above a large hotspot caused by the presence of a mantle plume. We infer that magmatism was initiated when rifting breached a large domal uplift supported by the plume-generated hotspot. The uplift-subsidence record in the Coppermine homocline is predicted by numerical models for rifting above hot asthenospheric mantle leading to ocean opening. An array of five large gravity anomalies north of the homocline may outline a region of stretched continental crust extensively intruded by Mackenzie mafic igneous bodies.

Geochemistry of basalts from the Dumisseau Formation, southern Haiti: implications for the origin of the Caribbean Sea crust

Basalt and diabase from the Cretaceous Dumisseau Formation, southern Haiti have Mg-numbers of 43–63, TiO 2 contents of 1.6–3.9% and La abundances of 3.6–15.3 ppm. La/Ta ratios average 10, and indicate that the basalts are oceanic in character, distinct from the arc associations forming the northern part of Haiti. Oldest lavas have low TiO 2 (1.6%) and are LREE-depleted, similar to N-MORBs, whereas overlying lavas have higher TiO 2 (2–3.9%) and are LREE-enriched, similar to E-MORBs or hotspot basalts. 87Sr 86Sr ratios vary from 0.70280 to 0.70316, 143Nd 144Nd from 0.512929 to 0.513121, and 206Pb 204Pb from 19.00 to 19.27. LREE-depleted lavas have high 143Nd 144Nd (0.51309–0.51310) typical of MORBs, whereas 143Nd 144Nd in the LREE-enriched lavas varies widely (0.512929–0.513121). Chemical features of the Dumisseau basalts are equivalent to those of Caribbean seafloor basalts recovered on DSDP Leg 15, and support the contention that the Dumisseau is an uplifted section of Caribbean Sea crust. Oldest lavas are analogous to MORB-like basalts cored at Leg 15 Sites 146, 150, 152 and 153, and the overlying lavas are analogous to incompatible-element-enriched basalts cored at Site 151 on the Beata Ridge. Isotopic compositions of the Dumisseau basalts overlap with those of the eastern Pacific Galapagos and Easter Island hotspots. However, the presence of N-MORB basalts in the lower part of the Dumisseau and at the majority of Leg 15 Sites indicates that the anomalously thick Caribbean crust probably did not originate as a hotspot-related basaltic plateau, but may have been generated by on-ridge or near-ridge hotspot magmatism.

Possible perturbations to hotspot traces and implications for the origin and structure of the Line Islands

Perturbations to hotspot signals recorded on the surface of the earth will produce hotspot traces with characteristics more complex than those of simple time‐progressive volcanic chains. Some of these perturbations are a consequence of normal plate tectonics, some are observed interactions of hotspot magmatism with lithospheric structure and processes, and others are hypothetical. The Line Islands volcanic chain has a complex structure and some characteristics that cannot be reconciled with those expected of a simple time‐progressive hotspot trace. Two models for the development of the Line Islands are presented. Model 1 assumes that the Line Islands were formed between 130 and 67 Ma, and model 2 assumes an age span of 90 to about 15 Ma and incorporates the Tuamotu Islands as an extension of the Line Islands. In model 1 most of the complex structure and subsidence of the chain can be explained in terms of perturbations to the hotspot signal. Model 2 gives a good geometric fit to the volcanic chain but fails to explain the anomalous subsidence of the Line Islands.